1. Field of the Invention
The present invention relates to an alignable electronic background grid generation system for use with a computer graphics display.
2. Description of the Prior Art
Graphics display terminals are becoming widely used to display computer generated pictorial information. By displaying such information on a video cathode ray tube (CRT) display screen, the user can view the pictorial material essentially as soon as it is generated by the host computer. This is particularly advantageous when performing computer assisted design work, since changes in a design can be displayed pictorially as soon as they are made. Flexibility is enhanced by providing zoom and pan features which respectively allow "magnification" of the display and shifting or positional change in the portion of an image which is displayed.
For many applications, the presentation of a background grid as part of the video display is particularly beneficial. Such a grid may resemble graph paper, and serve as a background on which the pictorial information is overlaid. The grid provides a visible, dimensional reference for the pictorial display. By making every n.sup.th grid line (e.g., every fifth or tenth line) of higher intensity, the viewer's perception of overall position in the display is substantially enhanced. The dual intensity grid is particularly helpful when using the grid to measure larger distances. For many applications, grids of equal spacing between lines is preferred. For other applications, spacings which are function-related, such as logarithmic, are preferred.
Certain background grid techniques have been used in the past. The simplest is a physical overlay, typically in the form of a transparent plastic sheet having a ruled grid pattern, which sheet is physically mounted across the face of the CRT. Although providing a reference scale, such approach is particularly undesirable for several reasons. First, the scale is in no way aligned with the underlying picture. Thus, for example, to align the origin of a displayed graph or image with a particular grid intersection requires physical movement of the overlay or a shift in position of the entire video display, which may or may not be possible depending on the type of video generation scheme that is used. Secondly, the accuracy of measurement using the grid is limited by parallax. Further, the grid spacing, and its relationship to the edge of the video image, both are fixed. Therefore, if a different display scale or magnification is used, or if the image position is moved or panned, a completely different transparent overlay grid may be required, or the position of the overlay must be moved to compensate for such image panning.
Certain electronic background grid generation schemes also are known in the prior art. For example, the grid itself may be generated and stored as an integral part of the displayed image. This requires the computer directly to generate the entire grid information as it is generating the desired pictorial image, to integrate these with one another, and to store the resultant combined pictorial image and grid background as a unitary pattern in a display memory. Although this eliminates the problems of parallax and grid-to-picture misalignment, it introduces other complications. For example, the grid will be presented with the same intensity as the pictorial image. As a result, rather than aiding interpretation of the image, the equal intensity background grid may well obscure the pictorial data. Although this problem can be cured by storing intensity information for the grid in the display memory, that approach would require the entire display memory to be capable of storing multi-bit intensity information for each individual picture element ("pixel"). The computer must generate separate intensity information for each grid and pictorial image pixel. Obviously, greater computer speed and complexity is required, and the display memory itself is more complex and hence of higher cost.
In addition, this prior art approach is not readily amenable to changes in grid scale factor or grid offset from the edge of the display. If a different scale factor or offset is desired, the entire image, including both grid and pictorial data, must be totally recomputed.
An object of the present invention is to provide an electronic background grid generation system having the following features, among others:
1. The background grid is generated totally independently of the displayed pictorial image; yet
2. The grid is exactly aligned with the underlying picture;
3. Individual grid lines may be displayed with different intensities, e.g., so as to resemble drafting paper;
4. The grid has completely selectable scale factors, including selectable spacing between adjacent grid lines, and selectable control of the number of lesser intensity grid lines between each line of greater intensity;
5. The grid scale factors may be controlled automatically to conform with the pictorial image magnification factor in a zoom system, so that when a magnified display is selected, the background grid will automatically conform to the changed size of the zoomed image;
6. The grid offset from the side and top or bottom of the display screen can be separately controlled. This permits operator or computer selection of the spacing between the image edge and the first minor (i.e., lesser intensity) and first major (i.e., greater intensity) grid lines. This function enables the background grid to be maintained in exact alignment with an image as it is panned across the display screen;
7. Any grid line spacing can be implemented, so as to produce background grids of equal spacing or of functional spacing such as logarithmic;
8. Grid generation may be implemented either in real time or in non-real time, the latter approach substantially reducing the requisite high speed logic requirements without sacrificing display flexibility;
9. The background grid may be restricted to only a portion of the total displayed image, e.g., to leave a display area available for alphanumeric presentation without a background grid which would be objectionable in this region;
10. Different background grid patterns may be generated simultaneously in different zones of the screen; and
11. The background grid intensity and mixing functions can be chosen to permit the pictorial image to be seen through the grid.